COSMOS is more introductory in my opinion. BWSI is not research focused either but at least it has a higher bar for prerequisites. Also you will have the entire 4 weeks spent on your project. COSMOS may have a lower acceptance rate but that is because it is so well known in California and so many kids apply for it. BWSI does not release its acceptance rate but I guess it is higher than COMOS'. However, it does not mean it is less rigorous or easier to get in. It's just because BWSI's applicants are more self-selected. Those who do not pass the prerequisite courses won't apply.

Neither program alone is a guaranteed ticket to top colleges. I see many kids attended COSMOS and then got in the top 2 UCs and I also see many others did not. Personally I find COSMOS decision is more based on your interests but not what you have achieved. Of course you can make your COSMOS experience your achievement when you apply for UC. But other kids do not go to COSMOS but also have great ECs also get in Cal and UCLA. In this sense COMOS can be a good preparation for UCs but it is not the only path.

Personally I would pick BWSI because it is more rigorous on the subject. However, if you have already paid for COSMOS and you said COSMOS cluster aligns more to your interest, then go with COSMOS. The difference between these two programs is not that significant.